Anyone who took high school biology might have encountered the genus Drosophila. Also known as the fruit fly or the vinegar fly, the insect is prized by researchers because they have a simple, prolific reproductive cycle, making it easy to study many generations in a relatively short time.
Research on Drosophila has led to a better understanding of molecular mechanisms underlying human disease, as well as basic physiological, immunological, and neurological properties conserved between humans and these tiny insects. But the Drosophila has a complicated relationship with other organisms that ensure its fecundity and new research examines that relationship, which can go back hundreds of thousands to millions of years ago.
In newly published research titled Multiple origins of obligate nematode and insect symbionts by a clade of bacteria closely related to plant pathogens, Vince Martinson, an assistant professor in the Biology department at The University of New Mexico, and his colleagues Steve Perlman, Ryan Gawryluk, Brent Gowen, and Caitlin Curtis from The University of Victoria and John Jaenike from The University Rochester look at Drosophila neotestacea and its relationship with parasites and symbionts.
"Briefly, my research explores the interactions between animals (mainly insects) and beneficial microbial symbionts. Many invertebrates depend upon bacterial mutualists to survive ̶ known as obligate symbionts," Martinson explained. For example, aphids depend upon a bacterial symbiont (Buchnera) to provide essential amino acids that they cannot obtain from their diet of plant sap. Many of these fantastic stories about the relationships between insects and bacteria were written about by Ed Yong in his 2016 book I Contain Multitudes in which he shows how "ubiquitous and vital microbes are… how humans are disrupting these partnerships and how scientists are now manipulating them to our advantage."
"Actually, the current research published in PNAS expands upon the host-microbe interactions known in the Drosophila fly and the parasitic nematode Howardula system which was featured in I Contain Multitudes," he added. Normally flies that are parasitized by the nematode, which resides inside the fly abdomen, have fecundity near zero. However, the fly has acquired a bacterial symbiont Spiroplasma that specifically targets the nematode with a toxin, protecting the fly from the nematode. In this way, flies that harbor Spiroplasma have a much higher ability to produce live offspring if they are parasitized by the nematode.
In the current PNAS paper, Martinson and his colleagues find that the parasitic nematode Howardula also has a symbiotic bacterium that appears to be relatively young in relation to other host-symbiont relationships. This symbiont's function is currently unknown, but the experiments they performed indicated that the nematode cannot live without the symbiont. In other words, it is an obligate relationship.
"While most obligate symbiont-host relationships are ancient ̶ hundreds of millions of years ̶ we estimate this nematode symbiont to be very young ̶ less than 500 thousand years old. This provides insight into the early stages of host-symbiont evolution," Martinson explained. "Further, we found that this nematode symbiont is a member of a previously unknown, but very widespread group of invertebrate-associated bacteria. Within this bacterial group, obligate symbiosis with insect hosts has independently evolved at least three other times."
Martinson said the discovery of this widespread group of symbiotic bacteria is a first step in research toward better ways to fight disease in both humans and agriculture. Symbiopectobacterium are associated with insects that cause crop damage and blood feed on humans. Additionally, they are closely related to bacteria that cause soft-rot, which are massive plant pathogens worldwide. Future research on this bacterial group could identify 1) ways to control or prevent soft-rot bacteria from damaging crops and 2) ways to manipulate or kill invertebrate vectors of human and plant disease. Overall, microbial symbionts have incredible potential in biotechnology because they have evolved intimate relationships with insects and nematodes, both of which have outsized impacts on human and plant disease transmission, he remarked.
"We believe that our paper will be of broad interest to researchers studying symbiosis and pathogenicity. Symbiopectobacterium is an exciting model for studying the evolution of symbiosis, because it and its close relatives span the entire continuum of host-parasite interactions, from free-living, well-studied, genetically tractable pathogens, to cultivable facultative inherited symbionts, all the way to obligate symbionts. In addition, only a handful of obligate nematode symbioses have thus far been described," Martinson concluded.
Martinson VG, Gawryluk RMR, Gowen BE, Curtis CI, Jaenike J, Perlman SJ. (2020) Multiple origins of obligate nematode and insect symbionts by a clade of bacteria closely related to plant pathogens. Proceedings of the National Academy of Sciences. PNAS first published November 30, 2020; https://doi.org/10.1073/pnas.2000860117
